The present invention relates to a memory that includes a magnet packaged therein, and more particularly, to a perpendicular magnetic random access memory (MRAM) packaged with a permanent magnet to improve programming characteristics thereof.
Spin transfer torque magnetic random access memory (STT-MRAM) is a new class of non-volatile memory, which can retain the stored information when powered off. An STT-MRAM device normally comprises an array of memory cells, each of which includes a magnetic memory element and a selection transistor coupled in series between appropriate electrodes. Upon application of a switching current to the magnetic memory element, the electrical resistance of the magnetic memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
FIG. 1 is a schematic circuit diagram of an STT-MRAM device 30, which comprises a plurality of memory cells 32 with each of the memory cells 32 including a selection transistor 34 coupled to a magnetic memory element 36; a plurality of parallel word lines 38 with each being coupled to the gates of a respective row of the selection transistors 34 in a first direction; and a plurality of parallel bit lines 40 with each being coupled to a respective row of the memory elements 36 in a second direction substantially perpendicular to the first direction; and optionally a plurality of parallel source lines 42 with each being coupled to a respective row of the selection transistors 34 in the first or second direction.
The magnetic memory element 36 normally includes a magnetic reference layer and a magnetic free layer with an electron tunnel junction layer interposed therebetween. The magnetic reference layer, the electron tunnel junction layer, and the magnetic free layer collectively form a magnetic tunneling junction (MTJ). Upon the application of an appropriate current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction of the magnetic reference layer. The electron tunnel junction layer is normally made of an insulating material with a thickness ranging from a few to a few tens of angstroms. When the magnetization directions of the magnetic free and reference layers are substantially parallel, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ. Conversely, the electrical resistance of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel. The stored logic in the magnetic memory element can be switched by changing the magnetization direction of the magnetic free layer between parallel and anti-parallel with respect to the magnetization direction of the reference layer. Therefore, the MTJ has two stable resistance states that allow the MTJ to serve as a non-volatile memory element.
Based on the relative orientation between the magnetic reference and free layers and the magnetization directions thereof, an MTJ can be classified into one of two types: in-plane MTJ, the magnetization directions of which lie substantially within planes parallel to the same layers, or perpendicular MTJ, the magnetization directions of which are substantially perpendicular to the layer planes.
FIGS. 2A and 2B illustrate programming operations of an STT-MRAM cell including a perpendicular MTJ memory element 80 coupled to a selection transistor 82 in series. The MTJ memory element 80 includes a magnetic reference layer 84 having an invariable or fixed magnetization direction 86 perpendicular to the layer plane thereof, a magnetic free layer 88 having a variable magnetization direction 90 or 96 perpendicular to the layer plane thereof, and a tunnel junction layer 92 interposed therebetween.
FIG. 2A illustrates the writing process for switching the resistance state of the MTJ memory element 80 from high to low. As electrons that pass through the magnetic reference layer 84 are being spin-polarized, the spin-polarized electrons exert a spin transfer torque on the magnetic free layer 88. When the spin-polarized current or parallelizing current (ip) 98 exceeds a threshold level, the magnetic free layer 88 switches from the anti-parallel to parallel magnetization direction 90. It should be noted that the parallelizing write current (ip) 98 flows in the opposite direction as the electrons.
Conversely, FIG. 2B illustrates the writing process for switching the resistance state of the MTJ memory element 80 from low to high. As electrons pass through the magnetic free layer 88, the electrons with the same spin direction as that of the magnetization in the magnetic reference layer 84 pass into the magnetic reference layer 84 unimpeded. However, the electrons with the opposite spin direction are reflected back to the magnetic free layer 88 at the boundary between the tunnel junction layer 92 and the magnetic reference layer 84, causing the magnetization direction 96 of the magnetic free layer 88 to switch from the parallel to anti-parallel orientation when the anti-parallelizing current (iap) 100 exceeds a threshold level.
The voltages required to drive the parallelizing current (ip) 98 and the anti-parallelizing current (iap) 100 should ideally be similar in order to accommodate the control and power circuitry, which is normally designed and optimized for symmetric switching voltages. In the MTJ memory element 80, however, the magnetic reference layer 84 exerts an external magnetic field perpendicular to the layer plane thereof upon the magnetic free layer 88, causing the switching voltages to become asymmetric. Therefore, the stray magnetic field exerted by the magnetic reference layer 84 upon the magnetic free layer 88, also known as the offset field, needs to be eliminated or minimized to ensure symmetric switching behavior.
One approach for eliminating the offset field of the magnetic free layer 88 is to add one or more magnetic layers with fixed magnetization to the MTJ memory element 80 in order to counter-balance or cancel the stray magnetic field exerted by the magnetic reference layer 84. FIG. 3A shows an exemplary MTJ memory element 110 comprising the MTJ memory element 80 and a magnetic fixed layer 112 separated from the magnetic reference layer 84 by a non-magnetic coupling layer 114. The magnetic fixed layer 112 has a second fixed magnetization direction 116 that is perpendicular to the layer plane thereof and is substantially opposite to the first magnetization direction 86 of the magnetic reference layer 84. Another exemplary MTJ memory element 120 illustrated in FIG. 3B includes the MTJ memory element 80 and a magnetic compensation layer 122 separated from the magnetic free layer 88 by a non-magnetic spacer layer 124. The magnetic compensation layer 122 has a third fixed magnetization direction 126 that is perpendicular to the layer plane thereof and is substantially opposite to the first magnetization direction 86 of the magnetic reference layer 84.
While the magnetic fixed layer 112 and the magnetic compensation layer 122 may eliminate or minimize the offset field of the magnetic free layer 88 by counter-balancing the stray magnetic field exerted by the magnetic reference layer 84, the addition of the magnetic fixed layer 112 and the magnetic compensation layer 112 disadvantageously increases the total film stack thickness, which may complicate the etching and integration processes. Etching of magnetic material, which cannot be readily volatilized by chemical reactions with common etching vapors, is mostly a physical sputtering process that may redeposit sputtered magnetic material on the sidewall of the insulating tunnel junction layer 92 and cause the electrical shunting of the MTJ memory element. Therefore, the propensity for shunting of the MTJ memory element increases with increasing amount of magnetic material to be etched.
For the foregoing reasons, there is a need for an MRAM that has a desired symmetric switching behavior and that can be inexpensively manufactured with high yield.